NL8004153A - METHOD FOR PREPARING CARBONIC ACIDS FROM ALDEHYDES - Google Patents

Description

H.O. 29,128 N

V

Process for the preparation of carboxylic acids from aldehydes.

The present invention relates to a process for the preparation of carboxylic acids from aldehydes by oxidation with oxygen or oxygen-containing gases.

Because of the easy oxidative conversion of the aldehyde-5 group to the carboxyl group, aldehydes are preferred starting compounds for the preparation of carboxylic acids. The oxidizing agent used here is mainly oxygen in pure form or in admixture with inert gases. In order to avoid side reactions, the oxidation, which is not catalyzed or carried out in the presence of catalysts, is carried out at the lowest possible temperatures. Catalysts are predominantly suitable salts of the transition metals, in particular salts of cobalt and manganese as well as of chromium, iron, copper, nickel, silver and vanadium. Namely, the formation of carboxylic acids from aldehydes is associated with side reactions and decomposition reactions both in the presence and in the absence of catalysts. In such cases, the selectivity of the oxidation of the aldehyde to the corresponding carboxylic acid with the same number of carbon atoms can be significantly improved by adding alkali metal salts of weak acids. The disadvantage of this variant of the process is that the salts have an inhibiting effect, so that the reaction time must be extended for a complete conversion of the starting compound.

Therefore, the problem underlying the invention was to provide a process for the oxidation of aldehydes with oxygen or oxygen-containing gases to carboxylic acids which allows selective conversion of the aldehydes at the shortest reaction times.

The invention consists of a process for the preparation of carboxylic acids by catalytic oxidation of aldehydes with oxygen or oxygen-containing gases. The process is characterized in that the oxidation is carried out as a catalyst in the presence of the complex anion.

The process of the invention makes it possible to convert both straight and branched aldehydes with oxygen to acids having the same number of carbon atoms. Compared to the oxidation in the absence of the catalysts of the invention, only small amounts of by-products are formed and the reaction times, despite the low reaction temperature, are considerable 800 4 1 53

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2 shorter. Examples of aldehydes that can be reacted by the process of the invention are acetaldehyde, propionaldehyde, butyraldehyde, valeraldehyde, heptanal, further ariphatic aldehydes such as phenylpropanal, and aromatic aldehydes such as benzaldehyde. The process according to the invention is particularly advantageous for the oxidation of branched, preferably branched, aldehydes. The latter aldehydes are decomposed using known processes to form formic acid and formic acid esters.

The preparation of compounds containing the complex anion used as a catalyst is known. Thus, for example, the sodium salt Na ^ fPeCC ^ ^ HgO] is obtained from NajCFeCCN ^ NHj] by reaction with sodium hydroxide. Hexacyano compounds of iron can also be converted into compounds containing the pentacyano-aquo-anion (see Jimori, Z. anorg. Chem 167, 145 (1927)). Finally, it is possible to use complex iron compounds (Prussid compounds) containing five cyanide groups which form the pentacyano-aquo complex under the reaction conditions. These include, for example, compounds with the anion [?] E (CN), ZZ] ~ ~, where Z is ammonia or aromatic nitrogen heterocyclic radicals.

The catalytically acting anion is added to the aldehyde in an amount of 0.05-2% by weight based on the aldehyde. Depending on the nature of the aldehyde, it is only partially soluble in the reaction medium and is therefore also present as a heterogeneous phase when "used in relatively large amounts." However, it has been found that the undissolved complex iron compound is also catalytically active. and that catalyst concentrations at the upper limit of the stated range exert a beneficial effect.

The oxidation is carried out at temperatures of 20-50 ° C.

Since the catalyst is thermally unstable and since the decomposition products hardly have any activity, the use of higher process temperatures is not possible.

The reaction time depends on the reaction temperature and is, for example, in the oxidation of isobutyraldehyde 0.5-6 hours; at 30 ° C, the conversion is completed after about 2 hours.

The practical implementation of the reaction is based on the aldehyde with the catalyst dissolved therein and partially suspended in a suitable reactor, for example a tube-40 reactor provided with a stream of plates. The presence of a solvent is not necessarily required, however, in some cases, especially in the case of unstable aldehydes such as hydroxyaldehydes, to be effective. Suitable solvents are, for example, aliphatic or aromatic hydrocarbons or carbon fibers. The oxygen or the oxygen-containing gas mixture is passed through the aldehyde from below; a large excess of the oxidizing agent, especially when operating in the lower temperature range, is not harmful.

The method is illustrated in the following examples.

10 Comparative example

202 g (2.8 mol) of isobutyraldehyde were introduced into a 1-liter reaction tube equipped with a double jacket and a glass frit with oxygen being supplied. The temperature was kept constant at 30 ° C during the conversion. The degree of conversion was determined by checking the neutralization number.

After 5 hours, a neutralization number of 580 was reached. The total amount of by-products was about 4> 2% by weight.

Example I

The reaction was carried out under reaction conditions analogous to those of the comparative example, except that 4> 5 mmole tripotassium aquo-pentacyano-ferrate (II)

(corresponding to about 0.92 g of [FeiCN ^ HgO] ^ -) were added. After about 5 hours, the neutralization number reached the value stated in the comparative example. The total amount of by-products was only about 2.8% by weight. Example II

The reaction was carried out under reaction conditions analogous to that of the comparative example, with the difference that 1.2 mmol sodium iron (II) ammine pentacyanide-30 trihydrate as [Fe (CN 2 HgO] 3) forming compound. (corresponding to about 1.3 g [Fe ^ N ^ HgO] ^ ~) was added Already after about 2 hours the neutralization number reported in the comparative example was reached The total amount of by-products was only about 1.7 wt. %.

35 Other examples

Catalyst: Sodium iron (II) amine pentacyanide trihydrate as [FeiCN ^ HgO] ^ "forming compound.

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Claims (4)

1. Process for the preparation of carboxylic acids by catalytic oxidation of aldehydes with oxygen or oxygen-containing gases, characterized in that the oxidation of the aldehydes is carried out in the presence of the complex anion [Ee (CN) ^ H2 O] ^ as catalyst, 2. Process according to claim 1, characterized in that the oxidation of the aldehydes is carried out in the presence of compounds which form the complex anion 10 [Ee (CN) 5H 2 O] 5 "under the reaction conditions. 3. Process according to claim 2, characterized in that the oxidation of the aldehydes is carried out in the presence of sodium iron (II-aminepentacyanide trihydrate. 4. Process according to claims 1 - 3, characterized in that aldehydes containing 4-10 carbon atoms are used. 8004153